AG13, you don't have any backfat in that pic, you are being too critical of yourself!
But... that said, keep in mind that backfat is part and parcel of wedding gowns, it doesn't matter how thin you are.... I once went to a wedding where the bride was skeletal and SHE had skin pushing out over the top of the dress...- I am not kidding - her nerves got the best of her, and she lost too much weight and you could see every bone in her chest and back, and sunken cheeks and all - but still, the back... Google "why do all wedding dresses create back fat?" and then click "Images".... brides of every size and shape, all with one thing in common... but again, you look AWESOME in your dress, and I'm just not seeing what you're seeing. Try to pretend you're looking at pics of a friend in the same dress, and you'll see... all that's there is prettyness
Here's mine just ignore my face lol. Every time I look at these forums I have serious dress regret but I think once it's altered I won't. I already went to my first fitting and the mock bustle was beautiful!